The present invention relates to a method for driving an optical modulation device and more particularly a time-division or time-sharing driving method for driving an optical modulation device such as a display element, an optical shutter array and the like and especially for driving a ferroelectric liquid crystal device.
Liquid crystal display devices for displaying a pattern or information have been well known in which a plurality of scanning electrodes and a plurality of signal electrodes are arranged in the form of a matrix and liquid crystals are interposed between the scanning and signal electrodes, whereby a plurality of picture elements ("pixel") are defined. In order to drive such liquid crystal display devices, a time-division driving method is used in which an address signal is applied sequentially and periodically to the scanning electrodes and, in synchronism with the address signal, predetermined information signals are selectively applied to the signal electrodes in parallel. Such liquid crystal display devices and the methods for driving them have serious defects as will be described below.
A first defect is that it is difficult to increase the density of picture elements or the size of a picture. Liquid crystals which have been used in practice in liquid crystal display devices, because of their fast response and their low electric power consumption, are twisted nematic liquid crystals of the kind disclosed in, for instance, "Voltage Dependent Optical Activity of a Twisted Nematic Liquid Crystals", M. Schadt and W. Helfrich in Applied Physics Letters, Vol. 18, No. 4, (Feb. 15, 1971), pages 127-128. The liquid crystals of this kind assume a helical structure in which the molecules of a nematic liquid crystal having a positive dielectric anisotropy are twisted in the direction of the thickness of the crystal liquid and are arranged in parallel with each other between electrodes when an electric field is not applied. When an electric field is applied, the molecules of the nematic liquid crystal with a positive dielectric anisotropy are oriented in the direction of the electric field, causing optical modulation. In the case of a display device in which a liquid crystal of the kind described is used and which has a matrix of electrodes, a voltage higher than a threshold voltage required for arranging the molecules of the liquid crystal in the direction perpendicular the surfaces of the electrodes is applied to a region (selected point) at which both a scanning electrode and a signal electrode are selected and no voltage is applied to a region (non-selected point) at which neither a scanning electrode nor a signal electrode is selected. As a result, the molecules of the liquid crystal are maintained in a stable state in which they are in parallel with the surfaces of the electrodes. When linear polarizers are disposed upon the upper and lower surfaces of a liquid crystal cell or device of the type described in cross nicol relationship, a selected point prevents the transmission of light while a non-selected point permits the transmission of light, whereby a display or picture is formed. However, in the case of a liquid crystal device with a matrix of electrodes, a finite electric field is applied to a region (the so-called "semi-selected or half-selected point") in which a scanning electrode is selected while a signal electrode is not selected or in which a scanning electrode is not selected while a signal electrode is selected. When the difference between a voltage applied to a selected point and a voltage applied to a half-selected point is sufficiently large, and if a threshold voltage at which the molecules of a liquid crystal are oriented in the direction perpendicular to an electric field applied is between the above described voltages, the correct operation of a display element can be ensured. However, when the number (N) of scanning lines is increased, a time period (duty ratio) during which one selected point is subjected to an effective electric field during the time when one frame is scanned is decreased at a ratio of 1/N. As a consequence, in the case of repetitive scanning, the greater the number of scanning lines, the smaller the effective voltage difference between a selected point and a non-selected point becomes. As a result, the problems of reduction in contrast of a picture and of crosstalk are unavoidable. These essentially unavoidable problems result when a driving method (that is, a repetitive scanning method) in which a liquid crystal which is not bistable (that is, a liquid crystal in which the molecules assume a stable state in which they are oriented in the horizontal direction relative to the surfaces of the electrodes and are oriented in the vertical direction only when an effective electric field is applied) is driven by utilizing a time storage effect. In order to overcome these problems, there have been proposed a voltage averaging method, a two-frequency driving, a multiple matrix method and so on. However, neither of these is satisfactory in solving the above described problems. Therefore, it is impossible at present to provide a display device with a large picture size and with a high density of picture elements because it is impossible to increase the number of scanning lines.
Meanwhile, a laser beam printer (LBP) in which the electrical signals representing a pattern are applied in the form of a light pattern to an electro-photographic sensitive member is most excellent as a means for obtaining a hard copy in response to the electrical input signals in the field of printers from the viewpoint of the density of picture elements and the copying speed. However, the laser beam printers have some defects as follows:
1. First, they are large in size as a printer. PA1 2. Second, they have moving parts such as a polygon scanner which are driven at high speeds so that noise is produced and these moving parts must be machined with a higher degree of dimensional accuracy.
In order to overcome the above and other problems, there has been proposed the use of a liquid crystal shutter array which is a means for converting electrical signals into optical signals. However, in the case of generating the picture-element signals with a liquid crystal shutter array, 2000 signal generators are needed in order to write the picture-element signals in a length of 200 mm at a rate of 10 dots per millimeter. Furthermore it is required to apply independent signals to these signal generators through respective lead wires. For these reasons, it has been difficult to provide a liquid crystal shutter array.
In order to overcome the above and other problems another attempt is made to apply one line of image signals in a time sharing manner by signal generators divided into a plurality of times. This method makes it possible to use a common electrode in order to apply a signal to a plurality of signal generators. As a result, the number of conductors can be reduced remarkably. However, when a liquid crystal lacking bistability is used and when the number (N) of lines is increased, the ON time of a signal is substantially reduced to 1/N. As a result, there arise the problems that the quantity of light incident on a photosensitive member is decreased and that crosstalk occurs.